1. Field of the Invention
The present invention relates to carriers used in developers for development of electrostatic latent images and, more particularly, to a carrier having magnetic powder dispersed in binder resin.
2. Description of the Prior Art
As developers for use with electrophotographic copying machines or printers, there have been known two-component developers composed of a toner and a magnetic carrier such as iron powder. In any developing method using such a two-component developer, the magnetic strength among carrier particles is so strong that ears of the magnetic brush harden, causing a problem that white lines may appear in black-solid images. Also, the iron powder carrier itself is low in volume electrical resistivity. Therefore, when the toner concentration in the developer has lowered due to continuous use or the like, electrical charges on the electrostatic latent image supporting member may escape via the carrier so that the latent image is disordered, causing defects or other damages in copy images, or electrical charges may be injected from the developing sleeve to the carrier so that the carrier adheres to the image portion. Further, if a hard carrier such as iron powder has adhered to the electrostatic latent image supporting member, the surface of the electrostatic latent image support member may be damaged when residual toner is removed.
To solve the above problems, a binder type carrier has been proposed in which magnetic fine powder is dispersed in binder resin. The binder type carrier is generally low in magnetization level within a magnetic field, compared with iron powder carrier or the like, so that the ears of the magnetic brush become soft. Thus, the binder type carrier has an advantage that excellent images free from white lines due to carrier can be obtained.
However, even with the use of the binder type carrier, especially when it is used in combination with a toner having a particle size as small as 3 to 9 .mu.m, there may arise some problems that the chargeability of toner is insufficient or the fluidity of developer is insufficient. Moreover, there may occur carrier adhesion that the carrier adheres to non-image portions of the electrostatic latent image supporting member, making image noise when developed, as still another problem.